The present invention relates to a method for aligning a wafer with a mask prior to transfering a circuit pattern from the mask to the wafer with a projection system during the photolithographic stage of an integrated circuit manufacturing process.
In the present state of the art of manufacturing integrated circuits, the means of transfering a circuit pattern from the mask to the wafer are frequently of the projection type. Such a system includes a condenser system for illuminating the mask and a projection system to transmit the light from the mask to the wafer. A wafer positioning or control system, which may include some or all of the lenses of the projection system, monitors the alignment patterns on the mask and wafer and positions the wafer to achieve alignment.
Dark field illumination is utilized in some kinds of automatic alignment devices to facilitate the sensing of the alignment patterns by heightening the contrast in the monitored images. A kind of dark field illumination system is taught in U.S. Pat. No. 4,301,363 wherein alignment is achieved by means of flying spot detection system. Light scattered during a scanning operation from the edges of alignment targets on the mask and the wafer is monitored by a photodetector via a viewing system containing a light intercepting plate, this plate implementing the dark field illumination.
A difficulty with conventional devices employing dark field illumination is the lack of selectivity: every edge or line on the mask or wafer is enhanced whether it belongs to the alignment targets or not. If the field of view of the alignment optics is sufficiently large to include other details on the mask or wafer, then some intelligence such as a computer program is required to distinguish the target patterns.
The recognition of the alignment patterns is facilitated when they are in the form of zone plates. As described in U.S. Pat. Nos. 4,037,969 and 4,326,805, zone plate alignment patterns, i.e., appropriately spaced concentric rings deposited or etched on surfaces of the mask and wafer, focus incident light in the respective zone plate focal planes, the focused light patterns being directly monitored by the alignment system.